For a variety of reasons most electronic systems are modular in design. Different aspects or functions of a system are implemented on different circuit boards, thus allowing systems of varying degrees in functionality to be easily put together by including or excluding certain circuit boards. Modularity also allows systems to be easily upgraded with additional functions by adding or replacing certain circuit boards. Often, one of the circuit boards is considered to be the main circuit board, to which all the other circuit boards are connected. The main circuit board is often referred to as the backplane or the motherboard, whereas the other circuit boards are often referred to as the add-on boards/cards or daughter cards. For example, in the case of personal computers the circuit board where the main components such as the processor and the memory are mounted is often considered the motherboard. The expansion or optional functions, such as serial/parallel interfaces, game adapters, graphics and/or video adapters, network adapters, and modems, are implemented using add-on/daughter cards, which are connected to the expansion slots or connectors of the motherboard. However, in recent years, the role of the expansion slots/connectors is beginning to blur, as manufacturers begin to package processors and memory components on daughter cards, also referred to as processor cards.
A result of the increased heat dissipation and electromagnetic interference (EMI) shielding requirements of the newer high speed processors, continuous improvement in processor card design has been a goal of computer system designers.